The present invention relates to a method of feeding a subliming, particulate material into a liquid. More particularly, this invention relates to a method of feeding a subliming, particulate material into a molten salt electrolysis cell.
An example of such a cell is disclosed in U.S. Pat. No. 3,822,195 of Dell et al. issued July 2, 1974, for "Metal Production". The cell includes an anode, at least one intermediate bipolar electrode, and a cathode in superimposed spaced relationship defining interelectrode spaces. It is utilized for the production of metal from the metal chloride dissolved in a molten solvent. The metal is produced by electrolyzing the chloride-solvent bath in each interelectrode space to produce chlorine on each anode surface thereof and metal on each cathode surface thereof. A flow of bath is established and maintained through each interelectrode space to effect removal therefrom of metal produced, this flow being such that it sweeps metal therewith out of each interelectrode space. In the operation of such a cell, additional metal chloride must be incrementally or continuously fed into the bath.
It is known from U.S. Pat. No. 3,135,672 of Hirakawa et al. issued June 2, 1964, for "Method for Feeding Alumina to Electrolytic Cell" to employ unconfined jet of gas in the feeding of alumina to an electrolysis cell which utilizes the commercially widespread Hall-Heroult process. In this process, aluminum metal is produced by electrolytic reduction of alumina in a bath of fused cryolite or fluoride. This process is in general characterized by higher temperatures than those encountered in the electrolytic reduction of aluminum chloride, and by the formation on the surface of the bath of a thick, hard crust. Hirakawa uses an unconfined jet of gas in feeding such a cell to set up a fine vibration or rippling on the surface of the bath, and thereby to prevent the formation of crust on the bath surface. With the formation of crust being thus prevented, alumina is fed continuously and constantly into the bath without any obstacles.
It is also known, as described in U.S. Pat. No. 2,713,024 of Mantovanello for "Process for the Continuous Feeding of Electrolytic Aluminum Cells", issued on July 12, 1955, to feed alumina into the bath under a continuous gas pressure sufficient to overcome the mechanical resistance of the superficial crust so that the alumina is forced to penetrate into the molten bath.
A method for feeding solid aluminum chloride to an electrolysis cell by dropping it on the bath surface is discussed in United States Department of the Interior, Bureau of Mines Report 6785, entitled "Electrodeposition of Aluminum from Fused-Salt Electrolytes Containing Aluminum Chloride". This report describes the appearance of a small puff of vapor as the charge of aluminum chloride comes in contact with the molten electrolyte surface. The appearance of this puff of vapor is an indication of sublimation of some of the solid AlCl.sub.3 to gas upon contacting the bath surface. Although such sublimation might not cause serious problems in an experimental situation (such as was the subject of the Bureau of Mines report), it presents problems that cannot be tolerated in a commercial operation. Such problems include the loss of availability of the AlCl.sub.3 that vaporizes to a gas on contact with the bath surface. In addition, this AlCl.sub.3 that has vaporized, or sublimed to gas, forms deposits upon reaching sufficiently cool surfaces, coating these surfaces and clogging orifices, etc.